1. Field of the Invention:
This invention is related to combination motor starters and more particularly to an integral modular motor controller wherein a single unit provides for complete protection and operation of a motor and associated circuitry over the entire current range.
2. Description of the Prior Art
Prior art combination motor starters as exemplified by U.S. Pat. No. 3,638,157 issued to Z. J. Kruzic are provided with separate units which provide for motor starting and stopping, current interruption, and current limiting. It is old in the art to provide a combination motor starter having a plurality of motor control and circuit protection devices connected in series in a common enclosure for complete motor and circuitry protection. It is old in the art to provide a fused switch motor starter combination comprising a contactor, with series connected overloads to automatically open the contactor during lesser overload current caused by motor overload, current limiting fuses, for interrupting high level fault current, and a manual operable switch, for permitting manual opening and closing of the motor circuit. It is also old to provide a fuse breaker combination motor starter comprising a contactor, with overload relays for opening the motor circuit upon the occurrence of lesser overload currents, a manually operable circuit breaker automatically operable to interrupt the motor circuit upon the occurrence of low level to high level fault currents, and current limiting fuses that operate upon the occurrence of high level short circuit currents. Prior art motor starters normally comprise a load-break device which usually has silver cadmium oxide, AgCdO, contacts or the like for low contact resistance. An electromagnet mechanism is used to open and close the contacts. Contacts are spring biased opened and are automatically opened if system voltage is lost. Low to high level fault current protection is provided by a series connected molded case circuit breaker. The molded case circuit breaker can be a single break deion circuit breaker with an adjustable magnetic trip. For long life and durability, silver tungsten contacts are often used in the circuit breaker. The adjustable magnetic trip can be set to operate the breaker from 7 to 13 times overload current. The molded case circuit breaker is capable of numerous operations interrupting high overloading currents. For example, the breaker used on a NEMA Size I starter having a 100 ampere rating can interrupt currents up to 15,000 amperes up to three times. Current limitation is provided by current limiting fuses connected in series with the contactor and molded case circuit breaker for protection against possible fault currents up to 100,000 amperes rms. In addition a thermal overload relay which can be adjusted to open the motor starter at currents below the molded case circuit breaker rating with an appropriate time delay is provided. It is also common to provide a series connected disconnect switch for isolating the other components.
In the prior art it is necessary to use a plurality of circuit protection and motor control devices to provide for complete protection and operation of the motor and associated circuitry over the entire current range.